1. Field of Invention
The present invention relates generally to the optical microscopes for viewing specimens, and more particularly to the illumination systems used to illuminate the specimens.
2. Description of Related Art
Optical microscopes are used to examine the microscopic structure of specimens in many diverse fields from metallurgy to biology and modern medicine. These diverse applications have differing needs to produce images observable through the microscope which show the features of interest in the specimen to be observed. The typical microscope includes a stand which supports a stage for holding a specimen placed on a glass slide, a light source for producing a light beam to illuminate the specimen for viewing, a collector for directing the light beam produced by the light source, a condenser which includes one or more lenses to concentrate the light from the light source through the collector to illuminate the specimen, an objective for receiving light from the specimen in the form of an image focusing an image of the specimen, the objective being vertically movable for focusing the image of the specimen, and an eyepiece for viewing the specimen directly using an observer's eye or a video camera for displaying the image on a monitor. Various conditioning devices may be inserted between the light source and the condenser such as filters to modify the image contrast such as spatial frequency, phase, polarization, and wavelength.
The specimen may be illuminated by transmitted light from the light source positioned behind the specimen held on the stage of the microscope which passes through the specimen forming an image of the specimen passing into an objective lens for viewing of specimens which are transparent or translucent as described above. Alternatively, specimens which are not transparent or translucent may be illuminated with incident light from a light source positioned on the same side of the specimen as the objective to bounce light off the specimen forming an image passing into the objective. A problem with both types of optical microscopes is that drops of liquids to be observed form a flattened hemispherical shape on a glass slide typically used to support specimens on the stage. The shape is basically the same as a hemisphere which is sliced upwardly from the bottom or largest diameter portion thereof. Therefore, the surface of the drop forms an angle with the slide typically greater than one-hundred-fifteen degrees. This angle deflects transmitted light away from the objective which causes a peripheral area of the drop in contact with the slide to appear as a dark ring which obscures viewing of the structure within the ring.
Koehler illumination was invented for illuminating specimens of microscopes to help microscopists to observe more the subtle features of specimens, including the dark peripheral ring of drop-shaped specimens. Koehler illumination utilizes, in addition to the components listed above for optical microscopes, an aperture diaphragm and a field diaphragm which are placed in the light beam produced by the light source ahead of the condenser. The aperture diaphragm allows adjustment of a numerical aperture of the light beam collimated onto the aperture diaphragm. The field diaphragm receives the light beam passing through the aperture diaphragm and allows adjustment of the light beam to illuminate an entire field of view of the microscope at the stage through the condenser. The subject and requirements of Koehler illumination is very technical and therefore is not discussed in detail herein. However, Koehler illumination is discussed in detail in the book “Microscopes: Basics and Beyond”, Volume 1, by M Abramowitz, available from the Olympus Optical Corporation, which book is herein incorporated by reference in its entirety.
Koehler illumination provides a number of advantages over non-Koehler illumination including: 1) a homogeneously bright field of view; 2) the working numerical aperture of the condenser may be controlled separately from the size of the illuminated field; 3) the specimen may be illuminated by a converging set of light wave fronts which maximizes lateral resolution and permits fine optical sectioning and maximum special resolution; 4) a front focal plane of the condenser becomes conjugate with a rear focal plane of the objective lens for optimal contrast enhancement of fine specimen details; and 6) flare arising from the microscope optics and the barrels such as that of the objective is reduced without any vignetting.
While Koehler illumination provides benefits, particularly in adjusting the lighting conditions such as improved adjustable image contrast so as to better view finer specimen details, optimal image quality, including optimal image intensity, resolution, and contrast, depends upon a number of conditions being satisfied. Unfortunately, most microscopists are not skilled enough to make the perfect adjustments necessary for optimal image quality such that Koehler illumination is not fully utilized, resulting in non-Koehler illumination being used. The image quality of Koehler illumination depends on perfect adjustment of: 1) the alignment of the illumination components; 2) the focus of the condenser onto the specimen; 3) the aperture opening of the field diaphragm; and 4) the match between the numerical aperture of the condenser and the particular objective used. Regarding the last item an optimal condenser has a numerical aperture greater than or equal to the numerical aperture of the associated objective. However, since most microscopes use a rotary device having multiple objectives of different magnifications. An objective having the desired magnification may be individually rotated into the light path to receive the image of the specimen for observation. However, a condenser of a corresponding numerical aperture must be used for optimal image quality. Since condensers are very expensive and time is involved to change the condenser each time the objective is changed, non-optimal condensers are often used with the objectives. Also, the adjustments are labor-intensive even for microscopists skilled in such adjustments, the goal being to align all of the optical components on a common axis of the light beam, to focus the various lenses correctly for Koehler illumination, and to adjust the aperture and field diaphragms appropriately.
Shortcomings of the Prior Art
While various devices have been patented which attempt to alleviate the problems of Koehler illumination, including the high skill level required of the microscopist, the time involved in setting up the microscope for Koehler illumination, and the high cost of having an optimal condenser for each objective, there are quite a few shortcomings. One such device is the illuminator elements disclosed in U.S. Pat. No. 5,734,498 issued to Krasieva, et al. on Mar. 31, 1998. The illuminator elements comprise minute fluorescent and/or light-scattering bodies suspended in a plastic polymer matrix. The illuminator elements replace the condensers of the optical microscopes to eliminate the need for an optimal condenser for each condenser. Illuminator elements of a fluorescent type are used with a light source of a type which causes the fluorescent bodies to glow. The illuminator elements are stated to provide improved image quality while eliminating the necessity of using an optimal condenser for each objective. However, the illuminator elements must be specially made out of a plastic polymer with the desired minute bodies, are expensive to produce likely using special processes to randomly scatter the minute bodies throughout the polymer matrix, are subject to damage by abrasives during use due to the relatively low hardness of plastic polymers, and some require a special light source which provides the necessary light frequency to provide fluorescence of the fluorescent bodies.
There is a need for a light diffuser for use with optical microscopes the manufacture of which utilizes off-the-shelf raw materials, requires no expensive special processes to manufacture, is made of a harder material such as glass which is less subject to damage by abrasives during use, and which uses the standard light source without changing to a special light source.